A solid oxide fuel cell (SOFC) operates at a high temperature of about 900 to 1000° C., and thus, exhibits superior electric power generating efficiency in comparison to other fuel cells. However, deterioration of fine structures of an anode, an electrolyte layer, and a cathode forming a unit cell caused by operation at high temperatures, restrictions in application of ceramic materials and an expensive manufacturing process bring about durability, reliability, and economic feasibility problems. As such, delays in practical utilization of SOFCs are prominent. Accordingly, in recent years, research and development is being conducted on reducing the operating temperature of SOFCs to a medium-low temperature of about 700 to 800° C. and in employing inexpensive metallic materials for an interconnector instead of expensive ceramic materials. A conventional SOFC unit cell operating at a high temperature of 900 to 1,000° C. is formed of an anode supporter, an anode reaction layer, an electrolyte and a cathode, wherein the anode supporter includes Nickel(II) oxide-Yttria-stabilized zirconia (NiO—YSZ), the anode reaction layer includes YSZ, and the cathode includes a Lanthanum strontium manganite (LSM) material so as to maintain mechanical properties of the ceramic unit cell.
Conventional SOFC unit cells are required to operate at a high temperature of at least 800° C. with the foregoing structure. Since output characteristics of an SOFC increase in proportion to an operating temperature, raising the operating temperature is favorable for efficient generation of electricity, whereas deterioration of the unit cell by a rise in temperature introduces durability issues and increased costs. Specifically, in an SOFC unit cell manufactured using a YSZ electrolyte material among conventional SOFC materials, substantial changes in an ohmic resistance occur according to the operating temperature. Particularly, a drastically increase occurs at an operating temperature of 800° C. or lower, for example, about 700° C., leading to a dramatic decrease in overall output characteristics of the SOFC unit cell. That is, the SOFC unit cell manufactured by conventional technology may exhibit low output performance, for example, about 0.35 watts/square centimeter (W/cm2) at an operating temperature of about 750° C.
Accordingly, there is a need to investigate design technology, new materials, and manufacturing processes for an SOFC unit cell which prevents a decrease in power output while maintaining an SOFC operating temperature of 800° C. or lower. That is, to resolve a decrease in output performance due to decreasing the SOFC operation temperature, use of Cerium (Ce) or Scandia Stabilized Zirconia (ScSZ) based solid electrolyte materials having excellent oxygen ion conductivity, instead of a conventional YSZ solid electrolyte, is being actively studied for reducing ohmic resistance by transferring oxygen ions in a unit cell. Also, a Lanthanum strontium cobalt ferrite (LSCF) material having excellent ion conductivity and electron conductivity is being researched for a cathode, instead of a conventional LSM material.
In the conventional technology, when the SOFC operating temperature is reduced to a medium-low temperature, electrochemical reaction properties are relatively deteriorated to increase the ohmic resistance of an electrolyte and electrochemical polarization resistance of a cathode, causing a considerable deterioration in output characteristics of an SOFC unit cell. Accordingly, rigorous studies are being conducted to derive an optimal output using the conventional materials. That is, manufacture of an SOFC unit cell which prevents a voltage drop due to a thinning electrolyte layer of a conventional YSZ material, employs novel solid electrolyte materials with excellent ion conductivity, such as Ce and ScSZ materials, and uses an LSCF material with superior conductivity, and catalytic performance for a cathode are being investigated.
There exists a design for an SOFC unit cell using ScSZ and Gadolinia-doped ceria (GDC)-based solid electrolytes having excellent ion conductivity and an LSCF material with superior electron conductivity. That is, designing and manufacturing an SOFC unit cell which uses conventional Ni—YSZ as an anode supporter, NiO—CeScSZ or NiO—GDC materials as an anode reaction layer, CeScSC or GDC materials as an electrolyte layer, and LSCF—CeScSZ or LSCF—GDC materials as a cathode is being examined. However, an LSCF cathode material reacts with a YSZ or ScSZ electrolyte to cause a dual-phase reaction on an interface between the electrolyte and the cathode, and thus, remarkably reducing output of the SOFC unit cell. Thus, suppression of such a side reaction is necessary to apply the LSCF material as a high-performance cathode material to the unit cell, and accordingly an interface film of a thin-film GDC electrolyte material is additionally needed between the cathode and the electrolyte. However, since the GDC material has poor sinterability and a higher sintering temperature than the ScSZ or YSZ materials, the electrolyte layer provides debased fineness after sintering acting as a new source ohmic resistance, thereby insignificantly improving practical output performance or rather reducing output characteristics according to circumstances and increasing manufacturing costs. Thus, an optimal design and an inexpensive manufacturing process for an SOFC unit cell based on convergence of the conventional technology and new material technology currently being developed is needed. Although manufacture of high-output SOFC unit cells operating at a medium-low temperature of about 750° C. has been recently researched and developed, an SOFC unit cell adopting a GDC buffer layer for controlling reaction between the YSZ electrolyte material and the LSCF cathode material involves a dual-phase reaction and process control problems.